NAME

ioctl - control device

SYNOPSIS

#include<sys/ioctl.h>intioctl(intfd,unsignedlongrequest,...);

DESCRIPTION

The ioctl() function manipulates the underlying device parameters of special files. In
particular, many operating characteristics of character special files (e.g., terminals)
may be controlled with ioctl() requests. The argument fd must be an open file descriptor.
The second argument is a device-dependent request code. The third argument is an untyped
pointer to memory. It's traditionally char*argp (from the days before void* was valid
C), and will be so named for this discussion.
An ioctl() request has encoded in it whether the argument is an in parameter or out
parameter, and the size of the argument argp in bytes. Macros and defines used in
specifying an ioctl() request are located in the file <sys/ioctl.h>.

RETURNVALUE

Usually, on success zero is returned. A few ioctl() requests use the return value as an
output parameter and return a nonnegative value on success. On error, -1 is returned, and
errno is set appropriately.

ERRORS

EBADFfd is not a valid file descriptor.
EFAULTargp references an inaccessible memory area.
EINVALrequest or argp is not valid.
ENOTTYfd is not associated with a character special device.
ENOTTY The specified request does not apply to the kind of object that the file descriptor
fd references.

CONFORMINGTO

No single standard. Arguments, returns, and semantics of ioctl() vary according to the
device driver in question (the call is used as a catch-all for operations that don't
cleanly fit the UNIX stream I/O model). See ioctl_list(2) for a list of many of the known
ioctl() calls. The ioctl() function call appeared in Version 7 AT&T UNIX.

NOTES

In order to use this call, one needs an open file descriptor. Often the open(2) call has
unwanted side effects, that can be avoided under Linux by giving it the O_NONBLOCK flag.

COLOPHON

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